The present invention relates to a charged particle beam apparatus, and more particularly to a charged particle beam apparatus having a function of properly adjusting the focus of an image obtained by charged particle beam irradiation.
A charged particle beam apparatus such as a scanning electron microscope is suitable for measuring or observing patterns formed on a semiconductor wafer, which has been becoming finer. Incidentally, samples used for such a purpose have taken a shape extending more three-dimensionally as semiconductor wafers have been multilayered. For example, currently, deeper contact holes have been formed in a sample.
A charged particle beam apparatus such as a scanning electron microscope thinly converges a beam and irradiates it onto a sample, which requires proper focusing of the beam on the sample. As semiconductor wafers have been multilayered, however, the distance between the surface of a sample and the bottom surface of a contact hole therein, for example, has become longer, causing a problem that the surface of the sample and the bottom surface of the contact hole have different focal distances. That is, focusing a beam onto the surface of the sample causes the bottom surface of the contact hole to be out of focus, producing a blurred image of the bottom surface of the contact hole.
Incidentally, Japanese Laid-Open Patent Publication No. 5-128989 (1993) discloses a technique which irradiates an electron beam onto a three-dimensional object while changing the focus of the beam, and extracts the contours of in-focus portions of the object to construct a three-dimensional image. In such an apparatus, however, when a two-dimensional image of a sample including the bottom of a contact hole is observed, for example, it is not possible to observe the details of the sample surface and the bottom portions of the contact hole since only the contour of the contact hole is indicated.
Japanese Laid-Open Patent Publication No. 5-299048 (1993) discloses another example which basically extracts contours of an object having concave/convex portions and produces a pseudo three-dimensional image in a representation similar to a contour chart.
The technique disclosed in Japanese Laid-Open Patent Publication No. 5-299048 (1993) performs differential processing on an image obtained by changing a focus, and extracts portions whose differential values exceed a preset extraction level.
This process is repeated on a plurality of images obtained by changing a focus, and finally the extracted portions are combined to extract contours of concave/convex portions of the imaged object. At that time, no consideration is given to portions whose differential values are less than the extraction level. Furthermore, since the extraction level, which is an evaluation level for determining a contour, depends on the S/N ratio of an image and the shape of the object, it is not possible to set a constant value for all portions. When there are two types of concave/convex portions in an image as shown in FIG. 16, for example, since a shape 1601 has a steep inclination, its in-focus portion has a large differential value, while since a shape 1602 has a moderate inclination, its in-focus portion has a small differential value. Therefore, if the same extraction level is applied to both shapes, the shape 1602 may not be extracted, depending on a selected extraction level. Thus, failing to set an appropriate extraction level produces an unextracted contour portion. Although the example in FIG. 16 shows only two types of concave/convex portions, an actual image has an infinite number of concave/convex portions. It is impossible to set an extraction level by which all of these contour portions are extracted. Since the above example extracts contour portions of each image separately, and no consideration is given to relationships between images whose portions have been extracted, when the extracted portions are combined to produce a composite image without setting an appropriate extraction level, some portions in the composite image may be left indefinite, or portions extracted from two or more images may overlap, as shown in FIG. 17. That is, in the invention disclosed in Japanese Laid-Open Patent Publication No. 5-299048 (1993), it is very difficult to set an extraction level, and in addition, no consideration is given to a method for processing extracted portions between images.
It is an object of the present invention to obtain an image which is focused on all portions of a sample or a certain two-dimensional area of a sample and to provide a charged particle beam apparatus capable of obtaining a two-dimensional image which has no blurred part over an entire sample.
In order to achieve the above object, a charged particle beam apparatus in accordance with the present invention comprises a charged particle source, a scan deflector for scanning a charged particle beam emitted from the charged particle source on a sample, means for changing a focus of the charged particle beam emitted from said charged particle source, a charged particle detector for detecting charged particles obtained at a portion of said sample irradiated with the charged particle beam, and means for composing a two-dimensional image of the sample as viewed from a direction of said charged particle beam source, based on signals on which said charged particle beam is focused, said signals being among signals output from the charged particle detector.
With this configuration, it is possible to select charged particles emitted from a two-dimensional area of a portion in focus from among charged particles obtained from an entire sample, and use the charged particles to form a sample image. That is, since a sample image can be constructed based on charged particles focused on an entire area or a specific two-dimensional area in a beam scan area, it is possible to compose a two-dimensional image that is focused on the charged particle beam scan area or a specific two-dimensional area thereof.
Another mode according to the present invention utilizes differential values or changes in a Sobel value at same coordinates of a plurality of images obtained by changing a focus, and uses a pixel value of the original image of an image which has a maximum value of those values to compose an image. This eliminates setting of unstable parameters as well as overlapping of portions extracted from the same image or more than one image for composition, resulting in composition of a full-focused image.